The present invention relates to electronic data communications systems and, more particularly, to a novel method and apparatus for providing coherent digital data communications in non-coherent frequency-hopping systems.
It is now well known to employ frequency hopping (sometimes referred to as "spread-spectrum") techniques to reduce any deleterious effects of single-frequency or narrow-band interference in RF communications systems. For ease of implementation, frequency synthesizers are generally not designed to maintain phase continuity between hops; and the hopping supported by such synthesizers is called non-coherent frequency hopping. Communications systems with non-coherent frequency hopping usually employ modulation techniques capable of being non-coherently demodulated, such as MFSK (M-ary frequency shift keying) and DPSK (differential phase shift keying), although it is well-known that non-coherent modulation is less efficient that coherent modulation, if efficiency is measured in terms of the ratio E.sub.b /N.sub.o of bit energy E.sub.b to noise spectral density N.sub.o required to achieve a desired bit error rate (BER). Viterbi has theoretically shown (in "Spread Spectrum Communications--Myths and Realities", IEEE Commun. Mag., pp. 11-18, May 1979) that MFSK, for M&gt;16 and used in conjunction with error-correction coding, is about 3 dB inferior to BPSK (binary phase shift keying) with perfect phase tracking. As reported by S. Y. Mui and J. W. Modestino, in "Performance of DPSK With Convolutional Encoding on Time-Varying Fading Channels" (IEEE Trans. on Commun., vol. COM-25, No. 10, pp. 1075-1083, October 1977), when used in conjunction with convolutional coding, BPSK outperforms DPSK and 8-ary FSK, and DPSK is only marginally better than 8-ary FSK. Therefore, in theory and assuming that the channel phase shift can be estimated accurately, the required E.sub.b /N.sub.o ratio can be reduced by 2 to 3 dB if BPSK, instead of DPSK or MFSK, is employed in conjunction with error-correction coding. It is desirable to provide a method to use coherent BPSK modulation and convolutional coding in non-coherent frequency-hopping systems.
When data is transmitted by non-coherent frequency-hopping methods, the channel phase shift is independent from hop to hop. In order to demodulate a BPSK signal, it is necessary to estimate the channel reference carrier phase for each hop. Therefore, the key to realizing the efficiency of coherent BPSK is an effective phase estimation technique for non-coherent frequency-hopping transmissions.
The use of coherent modulation in non-coherent frequency-hopping systems could lead to bursts of channel errors. This comes about because a bad phase estimate could result in incorrect demodulation of a large fraction of the data bits transmitted within any one hop. It is well known that the performance of convolutional (Viterbi) decoders degrades if the decoder input errors are bursty. Therefore, to improve communications efficiency, it is essential to mitigate the effects of a bad channel reference carrier phase estimation on the decoder BER. It is known to use the mitigation technique of data interleaving, which acts to improve performance by breaking up bursts of errors and spreading the errors over the input sequence to the decoder.
It is desirable to provide even further mitigation of deleterious channel effects, so as to reduce the data bit error rate (BER). My invention provides, in such a communications system, a novel method and apparatus to better estimate the channel reference carrier phase in each of the plurality of successive frequency hops, as well as to weight the demodulated data output in accordance with a factor responsive to the confidence that the phase estimate for that particular hop is correct.